Diurnal variations in directional brightness temperature over urban areas through a multi-angle UAV experiment

摘要

Quasi-synchronous multi-angle observations of urban surface temperature (UST) are critical for understanding the urban thermal anisotropy (UTA) regime and validating UTA models. Such observations can be performed by thermal sensors onboard lightweight unmanned aerial vehicles (UAVs), due to their flexibility in sampling from different directions. This study designed a straightforward and efficient protocol to obtain quasi-synchronous multi-angle USTs based on a lightweight UAV for a typical residential area in Nanjing (China). By using this protocol, we retrieved a directional brightness temperature (DBT) dataset with 73 DBTs 12 times throughout the diurnal cycle. Our results exhibited that the diurnal UTA intensity (UTAI) well corresponds to that of air tem-perature: the maximum UTAI (14.0 C) occurred around 14:00 to 15:00 local time, and the minimum (3.5 C) occurred at night. Interestingly, the nighttime DBT variations were found to depend both on viewing zenith and azimuth angles, and a slight hotspot effect was identified. These findings notably differ from previous studies based on modeling, which often assumed the independence of the nighttime DBT of the viewing azimuth angle and the absence of the hotspot effect. The analogous pattern of UTA between the daytime and nighttime (especially before midnight) was seemingly driven by a remarkable temperature contrast among urban surface components that can persist for many hours after sunset. Thus, the designed observation protocol can be applied to retrieve quasi-synchronous multi-angle USTs for other types of urban surfaces, thereby facilitating a complete understanding of diurnal UTA variations and assisting the validations of UTA models.